1. Field of the Invention
The present invention relates to an activated carbon useful for an electrode as, for example, an electric double layer capacitor, a process for producing the activated carbon and an electric double layer capacitor employing the activated carbon. The electric double layer capacitor of the present invention can be widely used for power sources for various portable apparatus, standby power sources for domestic electrical equipment, UPS for optical communication, power sources for electric automobiles and the like.
2. Discussion of Background
As an electric double layer capacitor, a coin type obtained in such a manner that an element having a pair of electrodes consisting mainly of an activated carbon formed on a current collector and a separator sandwiched therebetween, together with an electrolytic solution, is sealed in a metal casing by means of a metal lid and a gasket insulating the casing from the lid, and a wound type obtained in such a manner that a pair of sheet electrodes is wound by means of a separator interposed therebetween to obtain a wound element, which is accommodated in a metal casing together with an electrolytic solution, and sealed in the casing so that the electrolytic solution does not evaporate from an opening of the casing, have been known.
Further, for an application which requires a large current and large capacitance, stack type electric double layer capacitor having an element obtained by stacking a large number of sheet electrodes by means of a separator interposed therebetween, incorporated therein, has been proposed (JP-A-4-154106, JP-A-3-203311, JP-A-4-286108). Namely, a plurality of sheet electrodes formed into a rectangle as positive electrodes and negative electrodes are alternately stacked one on another by means of a separator interposed therebetween to obtain a stacked element, a positive electrode lead material and a negative electrode lead material are connected with the respective terminals of the positive electrodes and the negative electrodes by caulking, and the element in such a state is accommodated in a casing, impregnated with an electrolytic solution and sealed with a lid.
Heretofore, as an electrolytic solution for an electric double layer capacitor, a solvent having a high dielectric constant such as water or propylene carbonate has been used so as to dissolve an electrolyte at a high concentration. As an electrode for an electric double layer capacitor, wherein the charge of an electric double layer formed on the surface of the activated carbon itself contributes to the capacitance of the electric double layer capacitor, made mainly of an activated carbon having a large specific surface area has been employed.
An activated carbon is produced usually by carbonizing and activating a carbon source derived from a plant such as a sawdust or a coconut shell, a carbon source derived from a coal/petroleum material such as a coke or a pitch, or a synthetic high polymer carbon source such as a phenol resin, a furfuryl alcohol resin or a vinyl chloride resin.
The carbonization means a sequence of processes, in which an organic carbon material as the above carbon source is heated and changed so that the carbon is concentrated through a bond-cleavage, a decomposition and a polycondensation to be converted to a solid carbon product (a carbonized product). The carbonization is carried out usually by heating a carbon source in a non-oxidizing atmosphere at a temperature of from 300xc2x0 C. to 2,000xc2x0 C.
The activation is a process in which a precise porous structure is formed in the carbonized product obtained above. Usually, the activation is carried out by a gas activation process in which the carbonized product is heated in a weak oxidizing gas containing carbon dioxide or water vapor at a temperature of from 500xc2x0 C. to 1,100xc2x0 C., so that the carbonized product is oxidized and exhausted to have a porous structure and to increase its surface area. Otherwise, the activation is carried cut by a chemical activation process (an alkali activation process) in which the carbonized product is mixed with an alkali metal hydroxide (such as KOH) in an amount of equal to or several times the mass of the carbonized product, and then the mixture is heated at a temperature of from the melting point of the metal hydroxide to 1,000xc2x0 C., preferably from 400 to 800xc2x0 C. in an inert atmosphere for from several tens minutes to 5 hours so that the carbonized product can have a porous structure and increase its surface area. The alkali metal hydroxide is removed by adequate washing after the activation process is finished.
As important properties required for an electric double layer capacitor, a) large capacitance, b) a high energy density, c) high durability when charging and discharging cycles are repeated, and d) a low internal resistance, may, for example, be mentioned.
As an electrode material presenting large capacitance among the properties, an activated carbon obtained by activating a carbon material derived from a pitch by heating in the coexistence of an alkali metal hydroxide (an alkali activation) has been proposed (JP-A-5-258996, JP-A-10-199767, U.S. Pat. No. 3,817,874, U.S. Pat. No. 4,082,694).
Further, it is reported that an activated carbon obtained by alkali activation of a carbon material having a relatively developed graphitizability, such as a pitch showing optical anisotropy i.e. so-called mesophase pitch as a carbon source, has large capacitance per mass, and has a relatively high bulk density. Accordingly, when the activated carbon is formed into an electrode, the electrode has a high density, whereby an electric double layer capacitor having the electrode has large capacitance per unit volume (JP-A-2-185008, JP-A-10-121336).
However, according to the studies of the present inventors, the electrode containing an activated carbon derived from the material of which the graphitizability is relatively developed has problems: the electrode incorporated into a capacitor cell case tends to expand remarkably at the process of charging, resulting in the failure of the cell case, moreover due to the expansion of the electrode the capacitance per volume of the electrode after expansion is not large enough comparing to that calculated from the volume of the electrode before expansion.
On the other hand, in an electric double layer capacitor, it is also known to use an electrode containing an activated carbon which has a large specific surface area, obtained by steam activation or alkali activation of a carbon source constituted by a carbon material having a relatively low graphitizability, such as a thermosetting resin for example a phenol resin or a pitch showing optical isotropy. This type of the capacitor has a large capacitance per unit mass, a low expansion of the electrode at the time of charging and a high durability when charging and discharging are repeated for a long period of time. However, this type of the capacitor has a problem that the capacitance per unit volume tends to be small because a bulk density of the activated carbon is low.
Although it has not been fully explained why an activated carbon electrode obtained by alkali activation of a carbon material having a relatively developed graphitizability expands during charging, the present inventors consider its reason as follows: In general, it tends to be difficult to increase the specific surface area of a carbon material having a developed graphitizability by gas activation. Accordingly, an alkali activation is employed to increase the specific surface area of the carbon material. The mechanism of the activation by an alkali metal hydroxide is not clearly understood in detail for the most part. However, for example using the example of an activation by KOH, KOH infiltrates into between carbon layers at a relatively low temperature of from 400 to 500xc2x0 C., during this step, carbonation of carbon and gasification of carbon with generated water or carbonic dioxide gas are caused, whereby carbon is consumed to increase the specific surface area, and a potassium metal generated by reduction of KOH is intercalated as a guest into between carbon layers (a host) (New Edition Activated Carbon, Yuzo Sanada, Kodansha Ltd. Publishers, Scientific).
It is estimated that in an activated carbon subjected to alkali activation has weak bonding force between carbon layers caused by the intercalation mentioned above. Accordingly, it is considered that when the activated carbon is used as an electrode for an electric double layer capacitor, not only ions are adsorbed into pores in the activated carbon but some ions are adsorbed into pores to widen the space between carbon layers at the time of charging.
It is an object of the present invention to overcome the above problems of prior art and to provide an activated carbon having large capacitance per unit volume when it is used as an electrode for an electric double layer capacitor, and a process for producing the same. Further, the present invention is to provide an electric double layer capacitor having large capacitance and high reliability wherein the expansion of an electrode is suppressed using the activated carbon as an electrode material.
As a result of carrying out extensive studies to solve the above problem of the conventional activated carbon, the present inventors have found the following facts: By mixing a carbon source to form high graphitizability carbon and a carbon source to form high non-graphitizability carbon, and, followed by subjecting the resulting mixture to a carbonization and an activation treatment can be obtained an activated carbon. Thus obtained activated carbon has an advantage of large capacitance per unit volume caused from large capacitance and a large bulk density of the activated carbon derived from a high graphitizability carbon component, and also an advantage of large capacitance per unit mass, a low deterioration in the performance after the repeating of charging and discharging cycle, and a low expansion of an activated carbon particle by ion absorption caused from an activated carbon derived from a high non-graphitizability carbon component. At the same time the above activated carbon further has property of complement to the weaknesses of the activated carbon derived from the other respective carbon-sources.
According to the present invention, there is provided an activated carbon for an electric double layer capacitor electrode, which comprises a stacking structure having 2 layers or less in a proportion of from 25 to 80% and a stacking structure having 5 layers or more in a proportion of from 2 to 30% in the distribution of a stacking structure as obtained by analysis of the X-ray diffraction pattern of (002) plane, and which has a specific surface area of from 500 to 2,800 m2/g and a total pore volume of from 0.5 to 1.8 cm3/g.
According to the present invention, there is provided a process for producing an activated carbon for an electric double layer capacitor electrode, which comprises: (1) a step of mixing a carbon source which forms a non-graphitizable carbon by heating (hereinafter also referred to as a non-graphitizable carbon) and a carbon source which forms a graphitizable carbon by heating (hereinafter also referred to as a graphitizable carbon); (2) a step of carbonizing the resulting mixture by means of heating; and (3) a step of activating the carbonized product.
Further, according to the present invention, there is provided an electric double layer capacitor comprising an electrode containing the activated carbon described above.